This invention relates to a non-destructive testing method for detecting long-term degradation of material of pipes, for instance catalytic reforming heater pipes as those used for reforming natural gas with steam, and more particularly to a method of ultrasonic flaw detection of pipes.
Conventionally, double probe type ultrasonic flaw detection of a pipe is made with the pair of probes disposed on the pipe as schematically illustrated in FIG. 6 of the accompanying drawings.
Thus, on outer surface of a pipe 1 defining a longitudinal axis thereof there are disposed a transmitter probe 3 and a receiver probe 4 to always be on a common peripheral circle normal to the longitudinal axis of the pipe 1 but movable as an integral assembly along the outer surface of the pipe 1 for scanning the surface. During such scanning, the transmitter probe 3 launches ultrasonic radiation towards material of the pipe 1, and the receiver probe 4 receives the ultrasonic radiation transmission echo as emerges out of the material of the pipe 1, to thus detect any flaw in the material of the pipe 1 from fluctuation of the echo.
With such conventional method, there is no serious problem in detecting flaws within the parent metal portion of the welded pipe, but it has practically been impossible to effectively detect flaws in an annular weld portion 19 since the transmitter and receiver probes 3,4 should then be placed on the weld bead and too heavy fluctuations appear as noises in the echo to properly detect flaws, because of irregular undulatious of the bead surfaces in the path of the ultrasonic radiation, namely at the points of entrance into and emerging out of the material of the pipe.